1. Field of the Invention
The invention herein relates to a fibrillated tape yarn.
It further relates to a high tenacity, low denier poly(ethylene terephthalate) fibrillated tape yarn and more particularly to a high tenacity, low denier poly(ethylene terephthalate) fibrillated tape yarn particularly suited for conversion into a sewing thread.
The invention also relates to an improved process for producing poly(ethylene terephthalate) fibrillated tape yarn.
Additionally, this invention relates to an extrusion die for the production of low denier fibrillated tape yarns and more particularly to an extrusion die which eliminates draw resonance in the extruded tape.
2. Prior Art
Synthetic yarns have found wide acceptance in the textile industry on the basis of their many superior properties when compared to natural fibers. Synthetic yarns, both from fibers and continuous filaments, are employed in the production of fabrics for the complete spectrum of end uses including apparel, home furnishing as, and industrial application.
Sewing thread, however, which is used in almost all the processes which convert textiles to a particular end-use product has, until recently, been exclusively a natural, and particularly a cotton product.
The spun cotton yarn dominated the sewing thread market because of its desirable bulk, pleasant soft feel in the fabric seams, a desirable appearance, and, most significantly its good sewing performance.
"Good sewing performance" is a general term which reflects the ability of a thread to sew smoothly and evenly, avoiding splintering, breakage, and snarls during the sewing process. While a multitude of undefined factors are believed to affect sewing performance, good loop formation, resistance to melting and/or fusion under conditions of use, and freedom from knots, slubs, and other irregularities are believed to contribute heavily to good sewing performance.
While exhibiting good sewing performance cotton yarns are not without disadvantages, though, such as relatively low strength, poor whiteness retention on exposure to heat, and an undesirable reaction to finishing chemicals.
Previous attempts to develop a sewing thread yarn with the advantages of cotton but without its drawbacks have not met with complete success. Poly(ethylene terephthalate) yarn has achieved some commercial acceptance as an alternative to cotton sewing thread but until the development of the yarn of this invention, poly(ethylene terephthalate) sewing threads exhibited disadvantageous properties of their own.
Previously known poly(ethylene terephthalate) staple sewing thread suffers from excessively high shrinkage requiring stabilization prior to use to avoid seam puckering as well as necessitating the same staple processing as cotton fiber, while poly(ethylene terephthalate) sewing thread produced from continuous filaments is too lean causing an undesirable sleazy feel and slippage in the fabric during the sewing operation resulting in seam puckering. Additionally, both versions of the prior art poly(ethylene terephthalate) threads are subject to fusion of the thread in some sewing operations.
By the invention herein it was discovered that a poly(ethylene terephthalate) fibrillated tape yarn could be produced which avoided many of the specific disadvantages of both the staple and continuous filament forms of poly(ethylene terephthalate) sewing thread and additionally exhibited good sewing performance.
In prior patent applications U.S. Ser. No. 72,710 of co-inventor Gibbon, filed Sept. 16, 1970, now abandoned, and U.S. Ser. No. 70,718 also of Gibbon, filed Sept. 9, 1970 and now U.S. Pat. No. 3,707,837 there is disclosed a process for producing a polyester fibrillated tape yarn by extruding a mixture of poly(ethylene terephthalate) and from about 0.1 to about 25% (by weight of polyester) of polypropylene into tapes followed by a two stage draw process and fibrillation, preferably with air operated false twisting jets.
Fibrillation processes and specifically those teaching the inclusion of an incompatible polymer to initiate fibrillation are known in the art. However, the successful application of these processes was limited to the production of polypropylene yarns. The applicant, as disclosed in the aforementioned applications, discovered that a fibrillatable poly(ethylene terephthalate) product could be produced by extruding a poly(ethylene terephthalate) tape with a minor amount of polypropylene dispersed throughout the tape and thereafter hot drawing at a temperature of from about 80.degree. to about 140.degree. centigrade and further hot drawing at a temperature of from about 120.degree. to about 230.degree. centigrade. This precursor tape could then be fibrillated to a yarn product by conventional mechanical fibrillation means including twisting, rubbing, brushing, gas or solids impingement, etc.
A particularly preferred fibrillation process is disclosed in the aforementioned U.S. Pat. No. 3,707,847 for fibrillating a fibrillatable tape at windup speeds in excess of 500 feet per minute. In this process the tape is subjected to the action of at least four fluid twisting means wherein the direction of twist imparted to the tape is completely and sharply reversed between adjacent twisting means and the tape is advanced from one fluid twisting means to another while being maintained under a tension of from about 0.05 to about 0.5 grams per denier.
While this process was successful in producing a poly(ethylene terephthalate) fibrillated product its application was limited to the production of yarns of relatively high denier (above 500) and intermediate tenacity (2-4 gpd), and hence this process would not produce the low denier, low denier per filament, high tenacity yarn which is particularly desirous for a sewing thread product.
Low denier fibrillated yarns could be achieved in principle by slitting a cast polyester film followed by drawing and fibrillation. However, casting a film thin enough to produce the desired denier per filament, which is controlled by film thickness, is extremely difficult to achieve. The difficulty is further compounded by the problems involved in slitting polyester film.
Production of low denier fibrillated yarn by the process disclosed in the aforementioned prior applications proved unsatisfactory because of the occurrence of draw resonance at the high melt draw down ratio necessary to produce a low denier product. Draw resonance is a phenonmenon which results in a cyclic variation in both the thickness and width of an extruded tape with the dimensional variations occurring in phase with each other.
When one extrudes then draws a thermoplastic shape into a quenching medium and increases the drawing speed continuously, the cross-sectional area of the extrudate, in the air gap between the die and coolant, will become smaller. At a critical take-up speed, a cyclic gauge pulsation appears in the extrudate and becomes more pronounced as the drawing speed is increased until it eventually breaks at the air-coolant interface. The cause of the oscillation is believed to be complex and related to drawing speed, air gap size, melt temperature and molecular weight of the polymer extrudate, among others. The only known method of prevention was to lower the melt draw down ratio or provide a method of cooling slower than the water quench.
Neither of these methods would prove satisfactory in the fibrillation process disclosed herein.
Production of a low denier fibrillatable tape with a lower melt draw down ratio would necessitate narrowing of the extrusion slit or reducing the slit length. However, it is impractical to manufacture a die with a slit less than about 0.005 inch wide. Even if such a die were able to be produced, it would rapidly clog during extrusion with particles present in the polymer melt. Reduction of the slit length produces thicker tapes which fibrillate to a coarse denier per filament making them undesirous for the end use contemplated here.
Rapid quench of the extrudate is necessary to the fibrillation process as more fully set forth hereinafter and thus eliminates slower cooling as a solution to draw resonance in the process herein.